During the period where the radiator is in an overheat condition or the radiator temperature is excessively high, if the radiator cap is detached for checking the radiator or for supplying a coolant the radiator, there is the danger that boiling water could be expelled or blown off from the fluid supply opening of the radiator. As a result a dangerous scald or the like could be inflicted. There is no conventional device which can prevent such a danger.